In polymer extrusion, e.g. in the production of polymer fibers and films (e.g. high grade optical film) via polymer extrusion, the molten polymer is filtered before the molten polymer is passed through the extrusion die. The filter has the function of removing impurities from the molten polymer and to shear the molten polymer in order to break down the gels in the molten polymer. In polymer fiber extrusion, such a filter is called a spin pack filter.
It is known to use a layer of sand placed on a filtration membrane, as is e.g. disclosed in U.S. Pat. No. 5,795,595.The layer of sand in the spin pack filter acts to shear the molten polymer. A drawback is that preferential channels are formed in the sand, resulting in unsatisfactorily shearing. The sand can be taken along by the molten polymer and cause quality problems and/or extrusion performance problems.
Alternative spin pack filters utilize a metal powder layer to shear the molten polymer, e.g. EP0455492A1 and WO12/004108A1.Metal powder layers have the drawback of having a low porosity and hence result in a high pressure drop of the molten polymer. Furthermore, a high pressure drop enhances slip through of gels, especially soft gels, which is negative for the quality of the produced polymer product.
WO2005/025719A1 discloses a spin pack filter comprising a porous structure of sintered short metal fibers having a polygonal cross-section. The short metal fibers act to shear the molten polymer. The short fibers have a length over diameter ratio between 30 and 100.The spin pack filter may comprise different fiber layers.
JP5253418A provides a sintered filter for molten polymer filtration. The filter is provided with a first filtration fiber layer made by laminating and sintering a linear metallic fiber of polygonal cross sectional shape, made by a machining or cutting method. The second filtration fiber layer is made by laminating and sintering curved fine metallic fiber of circular cross sectional shape, made by bundled drawing. The filter comprises an intermediate metallic fiber layer (positioned between the first and the second filtration layer) made by laminating and sintering a metallic fiber of fine diameter of polygonal shape made by machining or cutting. The sintered layers are each cut to shape and put on top of each other.
The short fiber layer of WO2005/025719A1 and the linear metallic fiber layer of JP5253418A are providing shearing properties to the filter. However, the fibers used in these shear enhancing layers of WO2005/025719A1 and JP5253418A do not allow to be handled as unbonded (e.g. unsintered) web panel. Therefore, webs with these fibers can be made, e.g. on a plate, and sintered. The size required for the filter can be cut out of the sintered layer. The size required for the filter is cut out of panels for the other layers.
The layers, with the size as required for the filter or spin pack filter, are put on top of each other. It is a problem that the manufacturing process of the spin pack filter is lengthy and complex.
It is also a problem that the filters of the prior art are not providing optimum gel shearing performance.